1. Field of the Invention
The present invention relates to a drum brake in which a parking brake is incorporated.
2. Description of the Related Art
Some drum brakes include handbrakes or parking brakes used when a vehicle is parked. Such a drum brake is shown, for example, in FIGS. 26 to 28.
The conventional drum brake is commonly referred to as a leading and trailing shoe design and includes a backing plate 3 for attachment to a nonrotatable component (not shown) of a vehicle and having a mounting portion 6 projecting outwardly therefrom. An anchor 8 is fixedly mounted to the mounting portion 6 by means of rivets 7. A pair of shoes (one is a leading shoe 1, and the other is a trailing shoe 2) each have one end in sliding contact with the anchor 8. Two wheel cylinders 10 have pistons (only one is shown) 9,9 and piston rods 11, 12 secured to the pistons. The other end of each of the shoes 1, 2 are in engagement with the piston rods 11, 12. When oil under pressure is supplied to the wheel cylinders 10, the shoes 1, 2 are moved outwardly from the anchor 8.
A rotary shaft S, such as an axle shaft, extends centrally through the backing plate 3 and has a front end connected to a brake drum 19 and a wheel (not shown). There are predetermined clearances between the inner periphery of the brake drum 19 and the outer surfaces of the shoes 1, 2. A braking force is generated when the brake shoes 1, 2 are forced tightly against the inner periphery of the brake drum.
Reference is now made to the structure of a parking brake. A strut 20 also extends between each of the other ends of the shoes 1, 2 and includes a self-adjusting mechanism designed for the automatic adjustment of the clearances between the brake shoes 1, 2 and the brake drum 19. A lever 21 has one end pivotably connected to the other end of the trailing shoe 2 and is engaged with the strut 20. The other end of the lever 21 is connected through a parking brake cable 23 to a parking brake lever mounted in the interior of the vehicle (not shown).
With this arrangement, the parking brake lever is operative to rotate the lever 21 through the cable 23. The strut 20 is then pressed toward the leading shoe 1. This forces the leading shoe 1 tightly against the brake drum 19.
As shown in FIG. 28, the self-adjusting mechanism includes an adjusting screw 25a extending from a rod 25 of the strut 20, and an adjusting gear 26a extending from another rod 26 and meshed with the adjusting screw. Rotation of the adjusting gear 26a causes the strut 20 to extend. An adjusting lever 28 is meshed with adjusting teeth formed on the adjusting gear 26a and is rotatable therewith when the shoes 1, 2 are forced against the brake drum under the action of a return spring 27 (see FIG. 26).
The prior art drum brake, however, includes no means for limiting the maximum angular position of the parking lever 21. It is for this reason that when the parking brake lever is excessively pulled, the lever 21 is rotated to come into contact the rotary shaft S. This results in the damage of the rotary shaft S and the formation of rust, or the lever 21 may be bent and malfunctions.